It is known in the art to simultaneously transmit two channels of light modulated signals over a single fiber in a fiberoptic communication system. For example, it is known to use a 1.3/1.5 micrometer wave division multiplexer (WDM) to combine or separate the two light modulated signals, whereby one is generated by a 1.3 micrometer optical source, and the other is generated by a 1.5 micrometer optical source, such as a laser diode, for example. The WDMs are broadbanded, and typically provide a passband of 1.310+/-20 nm and 1.550 +/-20 nm (nanometers). In such systems, the use of an exact optical source wavelength for each of the modulated light signals is not necessary, provided that the wavelength of such signals falls within the 40 nm window regardless of time and temperature considerations. Optical sources for providing the necessary light carrier waves, and the 1.3/1.5 micrometer WDM's have wide availability in the marketplace. Known techniques for accomplishing such multiple light signal transmission through a single fiber utilize two wide passband windows to eliminate the requirement to select source wavelengths. Such known systems provide high performance as required in telecommunication applications, and are relatively economic.
In present fiber optic communications systems, a significant increase in cost and complexity is encountered when adding a third channel or modulated light signal for bidirectional transmission through a single fiber optic cable. Adding a fourth channel makes such a system even more costly and complex. Where systems require high performance, the use of a third wavelength window, such as provided by a 0.8 micrometer optical source is not acceptable due to the performance being below that required even as a minimum for a high performance system. The only known option for adding third and fourth communication channels to such a system providing high performance has been to slice either the 1.3 micrometer window or the 1.5 micrometer window into two windows, and to employ narrowband filters to combine and separate the added channels. In turn, a further requirement is that the optical sources must be selected to have a precise wavelength, in order to match the narrowband filters passband, which must also be maintained with the passage of time and over a relatively wide temperature range. As a result, such known systems are expensive, and very complex.